Materials with high ratios of length-to-diameter, also referred to as high aspect ratio materials (HARMs), constitute an extensive group. Examples within this classification of materials include carbon nanotubes (CNTs), chopped carbon fibers, nanofibers, nanorods, nanobelts, nanowires, etc. The alignment of HARMs is of intensive importance for many applications, such as the synthesis of reinforced composites and high-quality electrical devices. The main goals of the alignment procedure are to exclude the anisotropic characteristics of the HARMs and improve their mechanical, electrical and thermal properties, as well as composites thereof, in one special direction.
Detailed literature review indicates that most of the existing alignment methods have limitations, e.g., they are subject to poor alignment, specific matrix materials, or small-scale matrices. The application of electric fields has been studied to align a number of HARMs in a matrix between two electrodes. However, the operational drawbacks to this technique include, but are not limited to, high field strengths (e.g., about 1000 V/cm) and frequencies (e.g., 10-100 MHz). Therefore, these latter approaches are costly and restricted from widespread application.
Magnetic fields are also used for aligning a number of HARMs along the field lines. Although this method can properly orientate the HARMs, the technique demands strong fields (15-25 Tesla) for almost all HARMs due to their poor magnetic susceptibility, which increases the costs and causes the method to become infeasible for industrial applications.
Several efforts have been made for the attachment of magnetic particles to the HARMs in order to enhance their magnetic attraction. It is possible to attach magnetic nanoparticles to the surface of carbon nanotubes (CNTs) to elevate the above-mentioned susceptibility and hence align the HARMs by low-strength magnetic fields. A major disadvantage of this method relates to the residual of the applied magnetic nanoparticles in the final product, which imposes some undesirable properties, such as the materials being overweight (e.g., 100 to 200 times higher than the weight of the pristine HARMs). Moreover, an appropriate attachment of the magnetic nanoparticles depends upon the superficial chemical characteristics of the HARMs, which is not always possible.
Another recent method for alignment of CNTs under the influence of low magnetic fields involves the adsorption of magnetic nanoparticles onto the surfaces of the CNTs due to the use of surface-active agents or surfactants. The electrostatic attraction caused by the so applied surfactants forces the magnetic nanoparticles to aggregate upon the surface of the CNTs and consequently increase their magnetic susceptibility. The final product of this method, however, suffers from the aforementioned problem of high density, as well as other unwanted properties due to the magnetic particles. In general, both the physical and the chemical attachment processes entail either expensive, time-consuming or destructive procedures.
In other research, an attempt to increase the magnetic susceptibility of CNTs was made by loading them with paramagnetic iron oxide nanoparticles. As is understood in the art, to embed the magnetic nanoparticles, CNTs must have open ends. Generally, opening the ends of CNTs causes the destruction of the graphitic structure of CNTs, which, in turn, results in the reduction of the mechanical properties of CNTs. Also, this approach is limited only to nanomaterials possessing tubular/void configurations in order for loading the magnetic nanoparticles.
In conclusion, the current, existing alignment methods, based upon the application of magnetic fields, could be divided into two categories. In the first one, strong magnetic fields are employed to achieve the desired alignment, which ultimately makes it infeasible from an economic point of view for industrial applications. The second category pertains to methods which involve the use of the magnetic particles to compensate the need for magnetic fields of high strengths. These methods, however, are not able to effectively remove the added magnetic particles after the alignment process, and, consequently impose unfavorable properties onto the ultimate product.
It is, therefore an object of the present invention to provide an improved approach in the methodology for aligning HARMs, and improve the quality of the HARMs so produced.